The purpose of this project is to study immune responses in man and experimental animals to natural and experimental infections of Legionella pneumophila, the causative agent of Legionnaires' disease. This past year, we have compared the changes in pathogenicity, limits of infectivity in guinea pigs, behavior in tissue culture systems, and other variations of four strains of L. pneumophila when cultivated from embryonated hens' eggs and after numerous further passages on artificial growth media.